The Group H antigen has been identified as a cell wall gylcerol teichoic acid. It is obtained by the hot saline extraction method of Rantz and Randall and is distinct chemically from the lipoteichoic acid obtained by the phenol-water procedure. The latter antigen is associated with the cell membrane. The Group H antigen appears to be complexed or covalently bound to a polymer containing rhamnose, glucose and galactose and is chemically similar to an extracellular teichoic acid found in the spent medium. It is hypothesized that the latter polymer is derived by deacylation of the membrane lipoteichoic acid followed by linkage to the cell wall heteropolymer as it is synthesized. These modifications would make the teichoic acid better able to pass through the hydrophilic cell wall environment. In addition to the group antigen, a type specific antigen, b, has been purified by affinity chromatography. It is also a polymer of rhamnose, glucose, glucosamine whose major determinant appears to be an alpha 1,6-diglucosyl glucoside. The b antigen has been identified in dental plaque. Studies of adherence indicate strains containing this antigen adhere less well than strains in which it is absent. Thus the serological classification appears to be associated with important biologic properties of S. sanguis. Strains of S. mitis have also been studied and no LTA can be detected chemically or immunochemically. This characteristic appears to be an important criteria for identification of this species. Studies are continuing in an effort to define the specific antigenic determinant of the Group H antigen. It is possible that a better source of this antigen is the extracellular teichoic acid found in spent medium. In addition attempts are continuing to develop a serological grouping scheme for S. mitis. The hypothesis underlying this attempt is that studies of the cell walls suggest the presence of a ribitol teichoic acid in most strains. If this polymer is present it may be possible to produce an antiserum capable of detecting this component in many strains.